They ARE retarded, they're mutated people with no minds left to access, the reason there are special ones with powers is because those powers are unique and inherited, any Titan can gain it as long as it eats the previous owner of the power, it then gains access to it's own memories again and ability to turn human again

I saw season 1 and maybe I can explain. Titans have categories. There is one colosal, then there are titans who are intelligent and have special abilities (like armored one, Eren who regenerates and Female titan who does martial arts and is fast af). So there is a type of titans who are autistic and very unpredictable with their behavior. Their bodies are deformed and proportions are strange.